In many security systems, PIR motion detectors are distributed within a monitored area to detect intruders, e.g., for commercial or home security applications. A PIR motion detector is typically mounted on a wall, in a comer of a room, or on a ceiling, to detect motion in a monitored area. A PIR motion detector includes a PIR sensor that generates a sensor signal representing a change in infrared energy, thereby providing an indication of intruder motion within the monitored area. The PIR motion detector typically includes an amplifier circuit that amplifies the sensor signal for analysis. The amplified sensor signal is analyzed by comparison to an amplitude threshold that corresponds to the presence of an intruder.
The output of a PIR sensor can be susceptible to drift, and can introduce dc offset to the sensor signal. Drift can result from environmental effects or component aging. Also, the level of dc offset produced by different PIR sensors can vary due to differences in manufacture or materials. With the introduction of significant dc offset, the amplified sensor signal can communicate inaccurate information relative to the pertinent threshold, causing false alarm triggering or inhibiting proper alarm triggering. Thus, substantial dc offset can cause false triggering when no intruder is present in the monitored area. Also, dc offset can boost sensor signals that ordinarily would fall below the threshold, e.g., signals generated based on the presence of pets or other small animals. In either case, the result is an inability to accurately detect intruders and take appropriate action.
To improve accuracy, PIR motion detectors typically incorporate ac-coupled amplifiers that eliminate dc offset from the PIR sensor signal prior to amplification. AC-coupling also serves to normalize the range of output levels produced by different sensors. The use of ac-coupled amplifiers drives up the cost and complexity of the PIR motion detector, however, due to the need for additional components. Also, ac-coupled amplifiers can be susceptible to "overshoot" problems that undermine the accuracy of the sensors. For example, an ac-coupled amplifier can suffer from overshoot when the PIR sensors generate a strong signal change, and particularly when the amplifier saturates.
Following a significant signal excursion, the amplifier must drive itself back toward a steady state condition. With insufficient damping, the amplifier can be driven beyond steady state and into an overshoot condition. For example, instead of returning from a negative signal excursion to a reference level, the amplifier output may substantially exceed the reference level. A substantial overshoot, when compared to the pertinent alarm threshold, can be misinterpreted as a security event. Consequently, like dc offset, overshoot in the sensor signal produced by an ac-coupled sensor signal can cause false triggering situations.